1. Field of the Invention
The present invention is related to a tunable multimode laser and, in particular, to a compact laser with mode-hopping wavelength scanning
2. Discussion of Related Art
External Cavity Lasers (ECLs) are now commonly used in several applications where a continuously tunable laser source is needed. Although there are many commercial manufacturers of ECLs, covering a broad range of designs on the market, there are still many applications that would benefit from a dramatic increase in the tuning speed that current designs cannot offer, even if this increase comes at the expense of other operating parameters of the system such as the dynamic coherence length and power stability.
More specifically, fast ECLs with tuning speeds greater than 1,000,000 nm/s, tuning ranges of at least 5% of the center wavelength, and coherence lengths of at least several millimeters are not currently available commercially. Lack of wavelength tuning speed is a major disadvantage in present ECLs in that there are many applications that would benefit from substantially increased tuning speeds. Current ECLs, for example, may have a sufficient tuning range and also a truly single-mode behavior, and thus a coherence length of significantly more than a few millimeters (often many meters), but their tuning speed is limited to approximately 1,000 nm/s or less.
Note that there are tunable lasers that offer rapid, highly precise wavelength switching. For example, Syntune of Kista Sweden offers one such switchable laser that is capable of point-to-point wavelength switching times of less than 50 ns. However, such systems do not offer continuous sweeping of their output wavelength. Instead, they move discretely from one wavelength to another without generating a well-defined wavelength during the time interval required to move from one stable wavelength to another. Further, the Syntune laser requires a significant manufacturing infra-structure and is not easily implemented at new wavelengths. Another product that has some of the desired characteristics is offered by Micron Optics. The Micron Optics system is referenced in a scientific article that appeared in Optics Express, 8 Sep. 2003, Vol. 11, No 18, pages 2183 to 2189. The Micron Optics source provided 2 mW of optical power, a sweep time of 3.5 ms, a center wavelength of 1308 nm, and a FWHM (full width half maximum) sweep bandwidth of 87 nm. The Micron Optics system, assuming that the total swept range (defined as the 99% power points) is approximately twice the FWHM sweep bandwidth of 87 nm, results in a sweep speed of approximately 50,000 nm/s. The Micron Optics system, however, utilizes an intra-cavity fiber optic element and is susceptible to unwanted polarization variations as well as being limited to tuning speeds much less than that required by high-speed applications. Additionally, the Micron Optics system is a fiber based laser and as such it is difficult to make the cavity sufficiently short to enable very high tuning rates. The length of a tunable laser determines the transit time of a photon or a group of photons. If a group of photons of a specific wavelength takes too long in transiting the laser, when they return to the filter element the filter element could be tuned to another wavelength and hence provide unwanted attenuation of the laser action. The article by R. Huber et. al. in Optics Express 2 May 2005 V13, No 9 page 3513 to 3528 provides a more complete description of this limitation.
The typical design solution for an ECL is to provide a mirror or grating rotatable around a pivot axis that can be rotated to hold the laser in the same longitudinal mode over the laser's entire tuning range. The pivot axis is chosen such that both equation 1 and equation 2, shown below, can be simultaneously satisfied:λN=d(sin α+sin β),  (1)NλN=2L.  (2)In Equations 1 and 2, λN is the average instantaneous output wavelength of the laser, N is the longitudinal mode number of the laser, d is the grating constant measured in the same units as the wavelength, α is the angle of incidence of the light field upon the grating, β is the angle of diffraction of the light field off the grating, and L is the optical path length of the laser cavity. If the two conditions of Equations 1 and 2 are simultaneously fulfilled during the wavelength tuning with N fixed, the resulting laser will tune continuously and without any longitudinal mode hops. While there are several mechanical solutions that may satisfy the requirements of the two equations shown above, those that are found commercially or in the scientific literature are limited, due to their size, to tuning speeds significantly below the desired tuning speeds. The mechanics required to rotate a mirror or grating in accordance with the Equation 2 above with N fixed are almost always complicated mechanisms that have large inertial mass, which prevents them from being actuated at the high speeds required for rapid tuning.
To our knowledge, the fastest commercially available continuously swept single longitudinal mode tunable laser is offered by New Focus and is advertised as providing 1000 nm/s scan speeds. In order to reach the desired scan speeds of 1,000,000 nm/s, the moving optical element, which is typically a mirror or optical grating, is best rotated around its center of mass and is best kept as small as possible. Utilizing a resonant scanner provides a convenient means to achieve both these objectives; however this method typically has the rotation axis centered on the moving optical element, and Equation 2 cannot generally be met when N is held constant. In other words, the longitudinal mode condition NλN=2L cannot be satisfied continuously, and the laser radiation will jump from one longitudinal mode to another when the tuning range is significant enough to be of interest.
The mode jump can be one or several mode distances wide, the mode distance being:Δλ=λ2/2L. With these mode jumps present, the laser will no longer be a continuously tunable laser, and the dynamic coherence length of the laser will become erratic and degrade below the static coherence length of the system. In order to reduce the size of the mode jump, the cavity length can be increased, but that will also make the laser more disposed to multimode behavior and thus also contribute to a short coherence length.
In U.S. Pat. No. 5,956,355 (the '355 patent), a laser design in which the length of the cavity of a widely tunable single mode laser is adjusted to compensate for changes in wavelength is disclosed. The '355 patent disclosed that the laser could be made to provide nearly continuous frequency tuning through an appropriate choice of the laser cavity components and geometry while also offering a high scan rate. The method proposed in the '355 patent used a steerable mirror and a diffraction grating-which provided the wavelength selectivity-oriented such that the steerable mirror sweeps the light field across the grating in such a way to have the cavity length change to offset the changes in wavelength with the goal of maintaining a near balance in Equation 2 above, with N held constant. It was further proposed in the '355 patent that an additional element could be added such that the residual error or imbalance in Equation 2 could be accounted for such that a precise balance of Equation 2 could be maintained across a broad tuning range. While this approach seems feasible, to our knowledge, there has been no successful implementation of this proposed design.
The instantaneous coherence length of a wavelength swept laser can be measured utilizing one of several methods, usually involving different kinds of interferometers. One such method is to utilize a fiber based Michelson interferometer. The coherence length, Lc, is given by:Lc=2×HWHM, were HWHM is the displacement of one of the mirrors in the interferometer required to change the interferogram from 100% of maximum to 50% of maximum. Note that the factor of 2 accounts for the double pass (forward and backward) in the moving arm of the Michelson interferometer.
There is a need in many applications to simultaneously reach high nearly continuous-tuning speeds and dynamic coherence lengths of at least a few millimeters. This is the case in Swept Source Optical Coherence Tomography (SS-OCT) as well as in some optical remote fiber sensing and optical component testing applications. In SS-OCT the coherence length of the source sets a limit on the imaging depth, with the potential imaging depth scaling linearly with the coherence length.